it'll sound like you reversing into something cos ya can't see sh!t out the back //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
we don't actually help on here //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Good question on how it will sound. I don't think it will change a tuning frequency unless you have your cabin tuned, which would require that you already have a wall. I don't think it would change much but I am not positive. Good Luck
If you are competing, Id recommend against a rear wall. A wall will undoubtedly put you in some sort of unlimited, modified or even professional class. You will be competing against systems with walls located directly behind the driver's seat.
A wall mounted behind the rear seats does not produce any distinctive sound, if that's what you are asking.
I'll be building a 3rd row wall for 4 18's pretty soon. Only disadvantage I can see is you don't shrink down the cabin like a normal wall would. This is just a daily driver and he needs room for a BIG amp and bunch of batts, so this seems to be the best way...